This invention concerns a low-orbit satellite communications system for terminals, which may or may not be mobile and may be transmitter/receivers or just receivers.
Satellite communications with mobile terminals investigated up to the present use two types of orbit, namely geostationary satellite orbits and strongly inclined elliptical orbits, both with the property that they can be located, on the average, above the regions of space with strong concentrations of particles, known as Van Allen belts. Recently lower orbits have been considered, with altitudes located between 800 Km and 2000 Km. One characteristic of satellite communications systems which use such orbits is the possibility of communicating with a large number of mobile terminals, for example of portable type. However the poor radio performance of these terminals makes it necessary to provide compensation by way of superior radio performance of the satellite. The difference between the orbits at an altitude higher than the Van Allen belts and those at lower altitude lies in attenuation in space which is the less the closer the satellite is to the earth.
Depending on the frequency bands available there are two possible transmission systems. The first uses only the L band for the mobile-satellite links; the other uses the L and S bands. However, these systems can vary from country to country, since several transmission standards can be used, for example using TDMA access techniques, techniques of CDMA type, etc.
The communications system of the invention is compatible with "special mobile group" GSM standards. It is equally adaptable to CDMA transmission techniques. This system uses only one frequency band for the terminal-satellite links.
The object of the invention is thus to provide a particular transmission sequence, inasmuch as neither the satellite nor the terminal can receive and transmit at the same time. This makes it necessary to set up a transmission/reception frame and to establish special synchronization procedures, in time for a TDMA system and in time and in code for a CDMA system.
A CCIR Report (document No. US IWP 8/14-52, 1st Aug. 1990) entitled "Technical characteristics of a personal communication mobile-satellite system" describes a communications system by means of low orbit satellites with multi-beam antennas, each comprising 37 conical beams. Such a system suffers from the major problem of having a large number of beams which each form a small plot on the ground. Moreover changes in the beams can be produced with the passage of time, both because of the mobility of the users and because of the progression of the satellites. These are accompanied in general by a change of resources (handover). A high number of these in the course of a conversation spoils the quality of the link and affects listening comfort. Such a system has the disadvantage of a long transmission/reception frame.